The invention concerns a coater for application of coating color on a paper web.
Coaters of this general type are known in numerous variants, e.g.;
(1) DE 37 15 154 PA1 (2) DE 38 25 412 PA1 (3) DE 36 09 383 PA1 (4) DE 41 33 501
Problems are encountered with such nozzle applicators especially in the wedge-shaped entrance zone between applicator roll and backing roll, and particularly at high speeds. This is attributable most of all to the effect of air which is carried into the entrance gore along with the paper web as the backing roll rotates. The air mixes with the coating color at the point where the latter makes contact with the paper web. Occurring generally at the upper edge of the machinewide overflow slat is a crossing of the overflow stream, and at that, opposite to the travel direction of the paper web and opposite to the airflow. If the airflow impinges on this overflow stream, this results in a partial backup of the overflow stream and in the mixing of air and coating color.
It would be desirable for the paper web and coating color to form a straight line of contact with each other as they meet. Due to the illustrated effects of the air carried in, however, the line of contact is not straight, but frequently tongue-shaped. If there are permanent air inclusions which remain across the entire coating zone, the paper web is not wetted at all by coating color at the respective points, and thus remains uncoated at these spots. This is highly undesirable.
The problem underlying the present invention is to fashion a nozzle applicator so that the disturbing air inclusions in the entrance gore will be avoided. Specifically, the line of contact where the paper web and coating color stream meet is to be a straight line. Air inclusions are to be avoided at any rate.